A heat-sensitive recording paper is a recording paper which forms images by utilizing the physical and chemical changes of a material or materials induced by heat energy. Various processes have been investigated involving heat-sensitive recording papers.
Various recording members utilizing physical change or deformation of a material or materials by the action of heat such as the so-called wax-type heat-sensitive recording sheets have been known for a long time. Such materials are utilized for electrocardiograms. Examples of recording members utilizing chemical change by heat include one utilizing coloring or decoloring of a specific compound by heat and one utilizing the coloring reaction of two or more materials by the action of heat.
Examples of recording members utilizing the coloring reaction of two or more materials by heat include those involving a combination of a ferric salt of a higher fatty acid such as stearic acid and a polyhydric hydroxyaromatic compound (U.S. Pat. No. 2,663,654, 2,663,655, 2,663,656, 2,663,657, etc.), a recording member for forming azo dyes, oxazine dyes, etc., from dye-forming materials (Japanese Patent Publication No. 9240/63, etc.), and a combination of a colorless dye such as Crystal Violet lactone, and a phenol compound (U.S. Pat. No. 3,539,375).
These heat-sensitive recording papers involve first order coloration, i.e., they do not require development. Therefore, the recording means can be made light weight and miniaturized. These factors have recently made such recording paper desirable to utilize. However, such heat-sensitive recording paper cannot record at sufficiently high speeds. This is due to a limitation on the response speed of the recording element which utilizes heat energy. Various efforts have been made to improve the response speed by modifying both the recording apparatus and the recording paper. One effort involves increasing the smoothness of the surface of the heat-sensitive recording paper (as described in Japanese Patent Publication No. 20142/77 and Japanese Patent Application (OPI) No. 47351/73 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application")). In general, the smoothness of a heat-sensitive recording paper is increased by surface treatment such as super calendering, etc., whereby the heat transfer efficiency between the recording element and a heat-sensitive recording paper is improved which improves the recording speed.
However, improving the smoothness by such a surface treatment is accompanied by various faults. One of these faults is the occurrence of fog, that is, the occurrence of a coloring reaction in the surface treatment step, coloring the recording paper. Another method to increase response speed involves the addition of a granular wax as proposed in Japanese Patent Publication No. 14531/75. However, the wax generally has a large heat capacity and heat of fusion, which results in reducing the response time.
Another disadvantage of increasing smoothness is reduction in writability of the paper with a pencil, a ball point pen, etc. The reason for the reduced writability can be understood by considering that the action of these writing means depends on friction with the paper.
Still another disadvantage involves a problem on production step because there is a great reduction in the production efficiency since super calendering is performed by a separate machine.
When super calendering is carried out, the density of a heat-sensitive coloring layer is greatly increased. Accordingly, the dispersion of a heat-fusible material fused by a heat energy at recording is reduced forming adhesion between the recording element and the recording paper at the coloring portion. This reduces the running property of the recording element.
In spite of these faults, the surface treatment to increase the smoothness of a recording paper is presently performed since improving smoothness has a relatively large contribution with respect to improving response speed.
Japanese Patent Application (OPI) No. 24191/81 discloses a method reducing the base paper's beating extent below 250 cc (CSF) and increasing the bulk density above 0.9 by super calendering but the method is still insufficient to improve the recording speed, running property, and staining of head.